[JPA] merge()시 연관관계가 있다면 어떻게 동작하는가

 

 

 

(제 궁금증 풀 용도로 정리한거라 막 깔끔하게 정리하지는 않았습니다.)

 

 

 

 

🧐 ManyToOne, OneToOne 연관관계

일반적으로 merge 시 merge되는 특정 엔티티에 대해서만 조회합니다.
그러나 cascade 속성이 MERGE라면 merge 시 연관된 엔티티들도 left join으로 조회해옵니다.

merge()가 되어 값이 변경되는 것도 일반적으로는 fk 값의 변경만이 반영되지만,
cascade 속성이 merge라면 연관된 엔티티의 변경도 함께 반영됩니다.
(DB 관점에서 생각하면 편합니다)

@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Member {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    public Member(String name) {
        this.name = name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
}

@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Writer {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "member_id")
    private Member member;

    public Writer(String name, Member member) {
        this.name = name;
        this.member = member;
    }
    
    public void setMember(Member member) {
        this.member = member;
    }
}

 

 

 

🌈 일반 merge 시 발생하는 조회 쿼리

Writer만 조회하는 것을 알 수 있습니다.

 

 

 

 

 

 

🌈 cascade 옵션이 MERGE일 때, merge 시 발생하는 조회 쿼리

@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Writer {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.MERGE) // 추가
    @JoinColumn(name = "member_id")
    private Member member;

    public Writer(String name, Member member) {
        this.name = name;
        this.member = member;
    }

    public void setMember(Member member) {
        this.member = member;
    }
}

left fetch join으로 연관된 Member까지 가지고 오는 것을 알 수 있습니다.

 

 

 

 

 

 

 

 

 

🐳 대상 객체가 null이 되는 경우

결론: 연관관계가 제거됩니다.

@Test
void 대상_객체가_Null_이_되는_경우_연관관계가_끊어진다() {
    // given
    Member mallang = memberRepository.save(new Member("mallang"));
    Writer writer = writerRepository.save(new Writer("말랑", mallang));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setMember(null);
    writerRepository.save(writer); // merge
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.member()).isNull();
}

 

 

 

 

 

 

 

 

🐳 대상 객체가 다른 객체로 변경된 경우

결론: 연관 객체가 변경됩니다.

@Test
void 대상_객체가_다른_객체로_변경된_경우_연관관계도_변경된다() {
    // given
    Member mallang = memberRepository.save(new Member("mallang"));
    Member other = memberRepository.save(new Member("other"));
    Writer writer = writerRepository.save(new Writer("말랑", mallang));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setMember(other);
    writerRepository.save(writer); // merge
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.member().id()).isEqualTo(other.id());
    assertThat(byId.member().name()).isEqualTo("other");
}

 

 

 

 

 

 

 

 

🐳 id가 없는 객체로 변경되는 경우

결론: 예외가 발생합니다.
참고: cascade 속성이 MERGE인 경우 대상 객체를 저장하고 연관관계를 설정합니다.

@Test
void 대상_객체의_id가_없는_경우_예외가_발생한다() {
    // given
    Member mallang = memberRepository.save(new Member("mallang"));
    Writer writer = writerRepository.save(new Writer("말랑", mallang));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setMember(new Member("other"));
    assertThatThrownBy(() -> {
        writerRepository.save(writer);
        em.flush();
    }).isInstanceOf(IllegalStateException.class);
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.member().name()).isEqualTo("mallang");
}

 

 

 

 

cascade = MERGE인 경우

@Test
void 대상_객체의_id가_없는_경우_cascade_옵션이_MERGE_라면_저장되고_연관관계가_변경된다() {
    // given
    Member mallang = memberRepository.save(new Member("mallang"));
    Writer writer = writerRepository.save(new Writer("말랑", mallang));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setMember(new Member("other"));
    writerRepository.save(writer); // merge
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.member().id()).isNotEqualTo(mallang.id());
    assertThat(byId.member().name()).isEqualTo("other");
}

 

 

 

 

 

 

 

 

🐳 대상 객체의 일부 필드가 변경된 경우

결론: 일반적으로는 반영되지 않습니다.
참고: cascade 속성이 MERGE인 경우 반영됩니다.

@Test
void 대상_객체의_일부_필드가_변경된_경우_반영되지_않는다() {
    // given
    Member mallang = memberRepository.save(new Member("mallang"));
    Writer writer = writerRepository.save(new Writer("말랑", mallang));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    Member member = writer.member();
    member.setName("변경말랑");
    System.out.println("변경하려는 이름: " + writer.member().name());
    writerRepository.save(writer); // merge
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.member().name()).isEqualTo("mallang");
 }

 

 

 

 

 

cascade = MERGE인 경우

@Test
void 대상_객체의_일부_필드가_변경된_경우_cascade_옵션이_MERGE_라면_반영된다() {
    // given
    Member mallang = memberRepository.save(new Member("mallang"));
    Writer writer = writerRepository.save(new Writer("말랑", mallang));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    Member member = writer.member();
    member.setName("변경말랑");
    System.out.println("변경하려는 이름: " + writer.member().name());
    writerRepository.save(writer); // merge
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.member().name()).isEqualTo("변경말랑");
}

 

 

 

 

 

 

 

 

 

 

 

🧐 OneToMany 연관관계 (One이 연관관계의 주인이 아닌 경우)

@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Writer {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToMany(mappedBy = "writer")
    private List<Post> posts = new ArrayList<>();

    public Writer(String name) {
        this.name = name;
    }

    public void setPosts(List<Post> posts) {
        this.posts = posts;
    }

    public void addPost(Post post) {
        post.setWriter(this);
        posts.add(post);
    }
}


@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "writer_id")
    private Writer writer;

    public Post(String title) {
        this.title = title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public void setWriter(Writer writer) {
        this.writer = writer;
    }
}

 

 

 

 

 

 

 

🌈 일반 merge 시 발생하는 조회 쿼리

@Test
void 일반_merge() {
    // given
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    Writer writer = new Writer("mallang");
    writer.addPost(post1);
    writer.addPost(post2);
    Writer saved = writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writerRepository.save(saved);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");
}

 

 

 

 

 

@Test
void 일반_merge2() {
    // given
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    Writer writer = new Writer("mallang");
    writer.addPost(post1);
    writer.addPost(post2);
    Writer saved = writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    Writer find = writerRepository.findById(saved.id()).get();
    em.flush();
    em.clear();
    System.out.println("\n========[Find]========\n");
    
    writerRepository.save(find);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");
}

 

 

 

 

 

 

 

🌈 cascade 옵션이 MERGE일 때, merge 시 발생하는 조회 쿼리

@Test
void 일반_merge() {
    // given
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    Writer writer = new Writer("mallang");
    writer.addPost(post1);
    writer.addPost(post2);
    Writer saved = writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writerRepository.save(saved);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");
}

 

 

 

@Test
void 일반_merge2() {
    // given
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    Writer writer = new Writer("mallang");
    writer.addPost(post1);
    writer.addPost(post2);
    Writer saved = writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    Writer find = writerRepository.findById(saved.id()).get();
    em.flush();
    em.clear();
    System.out.println("\n========[Find]========\n");
    
    writerRepository.save(find);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");
}

 

 

 

 

 

 

 

 

 

 

🐳 List가 null로 변경된 경우

결론: 아무것도 달라지지 않습니다.

@Test
void List_가_null로_변경된_경우_아무런_변경이_없다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    writer.addPost(post1);
    writer.addPost(post2);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setPosts(null);
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
}

 

 

 

 

🐳 비어있는 List로 변경되는 경우

결론: 아무것도 달라지지 않습니다.

@Test
void 비어있는_List_로_변경된_경우_아무런_변경이_없다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    writer.addPost(post1);
    writer.addPost(post2);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setPosts(Collections.emptyList());
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
}

 

 

 

 

🐳 List를 비우는 경우

결론: 아무것도 달라지지 않습니다.
cascade 옵션이 MERGE이며, List를 비울 때 연관관계 참조를 제거하더라도 아무것도 변경되지 않습니다.

@Test
void List_를_비우는_경우_아무런_변경이_없다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    writer.addPost(post1);
    writer.addPost(post2);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    List<Post> posts = writer.posts();
    posts.clear();
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
}

@Test
void List_를_비우는_경우_cascade_MERGE_에_연관관계_참조를_제거하더라도_아무런_변경이_없다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    writer.addPost(post1);
    writer.addPost(post2);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    List<Post> posts = writer.posts();
    for (Post post : posts) {
        post.setWriter(null);
    }
    posts.clear();
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
}

 

 

 

 

 

🐳 List에 속한 엔티티 중 특정 엔티티가 달라진 경우(id 변경, 추가, 제거 등)

결론: 아무것도 변경되지 않습니다.

@Test
void List에_속한_엔티티_중_특정_엔티티가_달라진_경우_아무런_변경이_없다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    writer.addPost(post1);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.addPost(post2);
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(1);
}

 

 

 

 

 

 

🐳 List에 속한 엔티티 중 특정 엔티티의 일부 필드(id 제외)가 달라진 경우

결론: 아무것도 변경되지 않습니다.

@Test
void List에_속한_엔티티_중_특정_엔티티의_일부_필드가_달라진_경우_아무런_변경이_없다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    Post post1 = postRepository.save(new Post("제목1"));
    Post post2 = postRepository.save(new Post("제목2"));
    writer.addPost(post1);
    writer.addPost(post2);
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    List<Post> posts = writer.posts();
    for (Post post : posts) {
        post.setTitle("dd");
    }
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
    assertThat(byId.posts().get(0).title()).isEqualTo("제목1");
}

 

 

 

 

 

 

 

 

 

🧐 OneToMany 연관관계 (One이 연관관계의 주인인 경우)

@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Writer {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToMany
    @JoinColumn(name = "writer_id")
    private List<Post> posts = new ArrayList<>();

    public Writer(String name) {
        this.name = name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setPosts(List<Post> posts) {
        this.posts = posts;
    }

    public void addPost(Post post) {
        posts.add(post);
    }
}

@NoArgsConstructor
@Accessors(fluent = true)
@Getter
@Entity
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;

    public Post(String title) {
        this.title = title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

 

 

 

 

 

🐳 List가 null로 변경된 경우

결론: 연관관계가 제거됩니다.

@Test
void List_가_null로_변경된_경우_연관관계가_제거된다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    writer.addPost(new Post("제목1"));
    writer.addPost(new Post("제목2"));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setPosts(null);
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(0);
}

 

 

 

 

 

🐳 비어있는 List로 변경되는 경우

결론: 연관관계가 제거됩니다.

@Test
void List_가_비어있는_리스트로_변경된_경우_연관관계가_제거된다() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    writer.addPost(new Post("제목1"));
    writer.addPost(new Post("제목2"));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    writer.setPosts(Collections.emptyList());
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(0);
}

cascade가 MERGE가 아닌 경우

 

cascade가 MERGE인 경우

 

 

 

 

 

🐳 List에 속한 엔티티 중 특정 엔티티가 달라진 경우

결론: cascade, orphanRemoval 등의 옵션에 따라 여러가지 상황이 발생합니다.
(직접 확인하시길..)

 

 

 

 

🐳 List에 속한 엔티티 중 특정 엔티티의 일부 필드(id 제외)가 달라진 경우

결론: cascade가 MERGE 시 변경되고, 그렇지 않다면 변경되지 않습니다.

 

cascade가 MERGE가 아닌 경우

@Test
void List에_속한_엔티티_중_특정_엔티티의_필드가_변경된_경우() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    writer.addPost(new Post("제목1"));
    writer.addPost(new Post("제목2"));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    for (Post post : writer.posts()) {
        post.setTitle("dd");
    }
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
    assertThat(byId.posts().get(1).title()).isEqualTo("제목2");
}

변경되지 않는다

 

 

 

 

 

cascade가 MERGE인 경우

@Test
void List에_속한_엔티티_중_특정_엔티티의_필드가_변경된_경우() {
    // given
    Writer writer = writerRepository.save(new Writer("mallang"));
    writer.addPost(new Post("제목1"));
    writer.addPost(new Post("제목2"));
    em.flush();
    em.clear();
    System.out.println("\n========[데이터 저장]========\n");

    // when
    for (Post post : writer.posts()) {
        post.setTitle("dd");
    }
    writerRepository.save(writer);
    em.flush();
    em.clear();
    System.out.println("\n========[Merge]========\n");

    // then
    Writer byId = writerRepository.getById(writer.id());
    assertThat(byId.posts()).hasSize(2);
    assertThat(byId.posts().get(1).title()).isEqualTo("dd");
}

변경된다